The present invention relates to an automatic gas distributing device used in a gas supply system. The gas supply system has a gas source stored under high pressure in a plurality of high pressure vessels. Particularly, the present invention relates to an automatic gas distributing device which automatically exchanges an operational flow path from a high pressure vessel for that from another high pressure vessel on standby at full gas pressure without dropping the supply gas pressure during the exchange period.
In a chemical vaporizing deposition (CVD) fabrication process of a semiconductor device, various gases, such as silane (SiH.sub.4), hydrogen (H.sub.2) etc., are used. Generally, these gases are supplied to a fabrication apparatus under a tightly controlled predetermined gas pressure which is required to be maintained within a small variation range in order to perform the relevant fabrication process in a stable and reliable state. Accordingly, a drained high pressure vessel, which has a reduced gas pressure below a specified control pressure, will be exchanged for a new high pressure vessel on stand-by, which stores the gas at full pressure. During the exchange interval it is important to maintain a constant supply gas pressure so that no pressure drop of the supply gas is allowed throughout the entire exchange interval.
A prior art gas supply system is described with reference to the schematic diagram of FIG. 1. The system contains a gas distributing device for supplying gas continuously from two high pressure vessels 1 and 2 alternatively to a utilization pipe to be connected to a fabrication apparatus. The high pressure vessel, 1 or 2, is connected to, or disconnected from the system using a connecting valve 3 or 4, respectively. During the exchange interval, shut-off valves 5 and 6 are closed. The pressure of the gas (hereinafter, primary pressure) of the high pressure vessel 1 or 2, is regulated by a regulator 7 or 8, to a low pressure (hereinafter secondary pressure). The low pressure is the pressure at which the gas is utilized. The regulated gas is sent to utilization pipe (not shown), through a directional control valve 19 which is operated automatically or manually. Gas pressure indicators 11 and 12, are disposed upstream of the gas regulators 7 and 8, respectively. The gas pressure indicators 11 and 12 indicate the respective primary pressure. Downstream of the regulators 7 and 8, other indicators 13 and 14 indicate the secondary pressure. During manual operation of the device, the primary pressure of the high pressure vessels 1 or 2, is ready by an operator and one of the vessels 1 and 2 is selected by operating the directional control valve 19 after determining that the secondary pressure of the relevant vessels reaches the specified utilization gas pressure. During automatic operation of the device, the pressure indicators 11 and 12 are replaced by pressure signal generators, and the directional control valve 19 is replaced by an electrically or pneumatically driven directional control valve which operates in accordance with signals issued by the pressure signal generators. Thus, the drained high pressure vessel, having a primary pressure below a predetermined control pressure, is replaced by a full high pressure vessel. Consequently, the gas under the utilization gas pressure is supplied to the utilization pipe without any break or pressure drop. However, in the above-described automatic control system, an electrical and pneumatic power source are required to be used and therefore the system is more complicated and expensive than a manual system. In particular, when the power source is disabled, the operation of the entire gas supply system is stopped, which causes substantial damage to the fabrication process.
The above-described prior art gas distributing device has been occasionally been plagued by occasional misoperation and/or high labor operational cost of the operators. In order to overcome these disadvantages, various improved automatic gas distributing devices have been proposed. Most of them are of the type where the change of the secondary pressure of the gas is detected by deformation of a diaphragm member. The deformation is converted into an electrical control signal which is sent to the relevant control valves. However, the change of the secondary pressure can be very small, particularly, when the change of the secondary pressure is required to be strictly controlled as in the semiconductor fabrication apparatus. As a result, the regulation of the control signal is a very delicate operation which can decrease the accuracy of the control operation. Furthermore, if the relevant gas is corrosive, or explosive, the use of electrical components is rather undesirable in view of the safety and the reliability of the gas supply system.
One proposed automatic gas distributing device has a primary pressure of the gas source utilized as a power source of the device. The device is an automatic gas delivery device disclosed by Gerard Loiseau et al in U.S. Pat. No. 4,597,406 published on July 1, 1986. Usually, the change of the primary pressure of the high pressure vessels is large and advantageous to provide sensitive control signals for controlling the gas delivery device. In the automatic gas delivery device of Loiseau et al, various conventional pneumatic elements are utilized for detecting the primary pressure of the two high pressure vessels, one of which is in operation and the other of which is on standby. Gas flow coming from the high pressure vessel, which is drained below a predetermined pressure, is automatically exchanged for that from a full high pressure vessel on stand-by. The whole control system is driven by the aid of pneumatic controlling elements. Although there are advantages inherent to the device, such as the absence of an electrical power source, the source of a number of pneumatic elements are considered to make the device complicated and expensive.